This has to be open ended and you need to make a number of assumptions.
The dielectric strength of air is approximately 3 kV/mm. In other words, a pd of 3kV will produce a 1mm spark.
You need to estimate the pd between your body (hand) and the door knob. (Take the door to be zero)
Can you find an estimate for the capacitance of the human body?
If so, one estimate would involve using this and the charge given to find V.
However, the shape of the electrode (your hand) also plays an important role.